In particular in off-road vehicles, such as construction vehicles and tractors, the vehicle seat has a major effect on the comfort of travel. Excessive vibrations, impacts and movements which are transmitted from the road surface via the vehicle seat to the person located on the seat can lead to fatigue and in the longer term also to health problems, such as back pain or bone erosion, in the person. Usually, damping/suspension systems are used to reduce these effects. Various basic suspension variants are known for this purpose, which differ in complexity and energy requirements: passive, semi-active and active suspension systems. In passive vehicle seats, the properties of the system components are constructionally predetermined, and cannot be varied during operation. In semi-active suspension systems, the dissipating properties of the seat suspension can be influenced selectively depending on the situation at hand. The hardness of the damper can be adjusted and/or the rigidity of the spring can be adapted. In active systems, energy can additionally be introduced into the system by way of suitable actuators.
For constant amplitudes of the excitation, conventional seat suspensions have a relatively large increase in the vibration amplitudes of the seat surface in the range around the resonant frequency. This leads to amplification of the excitation and to a significant reduction in both the objective and the subjective comfort of travel. Considering the seat as a vibrating system, it can be assigned a natural frequency. If this system is excited by harmonic vibrations of the vehicle floor, it vibrates at the same frequency as the vehicle, but at a different amplitude. At excitation frequencies of 0 to √2 times the natural frequency of the vibration system, the vibrations transmitted to the seat are amplified, the amplification being at a maximum at the resonant frequency. A reduction in the introduced vibrations sets in above √2 times the natural frequency, and becomes greater as the excitation frequency increases. However, this effect is influenced by the damping of the system. The greater the damping, the less the reducing effect. By contrast, low damping of the seat leads to reduced seat comfort below √2 times the natural frequency.
To reduce the vibration amplitudes in the range of the resonant frequency, adjustable-force dampers and special control algorithms are generally made use of. The damping characteristic may be adjusted adaptively or semi-actively.
In adaptive adjustment, the amplitudes of the relative speeds are compared with one or more thresholds. If the amplitude of the relative speed is below the defined threshold, the damper is operated at a maximum force so as to suppress the increase in the vibration amplitudes in the resonant range. Above this threshold, the damper is virtually completely switched off so as to increase the travel comfort in the range above the resonant range. However, this type of adjustment exhibits considerable deficits in travel comfort if seat suspension is excited in the ranges above and below the resonant frequency simultaneously.
In semi-active adjustment, for example using the widespread Skyhook algorithm, the described drawback is overcome by using an additional movement variable, in this case the acceleration of the seat surface. The characteristic of the damper is adapted to the travel situation at hand as a function of the absolute and relative movement variables. Eliminating the increase in the vibration amplitudes in the range of the resonant frequency thus comes at the cost of installing an additional sensor for monitoring the absolute movement of the seat surface.